1. Field of the Invention
The present invention relates to a bismuth-substituted rare-earth iron garnet single crystal film suitable for a Faraday rotator used for an optical isolator. This single crystal film has a low saturation magnetization which lends itself to miniaturizing optical isolators.
2. Description of Related Art
Semiconductor lasers are widely used as a coherent signal source in the fields of optical fiber communications and optical instruments. However, semiconductor lasers are disadvantageous in that oscillation becomes unstable due to so-called reflected light return where light is reflected by, for example, the optical system back to the semiconductor laser. In order to address this drawback, an optical isolator is usually provided on the light-exiting side of the semiconductor laser. The optical isolator provides a light path which prevents the reflected light from returning to the semiconductor laser.
An optical isolator is usually constructed of two polarizers, a Faraday rotator, and a permanent magnet for causing the Faraday rotator to be magnetically saturated. Faraday rotators used for this purpose are usually bismuth-substituted rare-earth iron garnet single crystal films which exhibit large Faraday effect.
An optical isolator is built together with a semiconductor laser, in an integral construction referred to as a semiconductor laser module, and there has been a strong demand for a miniaturized optical isolator. Miniaturizing optical isolators requires a miniaturized cylindrical permanent magnet that is used for causing a Faraday rotator to be magnetically saturated. However, miniaturized cylindrical permanent magnets have weaker interior magnetic field strengths. Therefore, the Faraday rotator should have a low magnetic saturation so that the Faraday rotator can still be magnetically saturated by a weak external magnetic field.
In other words, in order that the permanent magnets can be miniaturized, a bismuth-substituted rare-earth iron garnet single crystal (referred to as BIG hereinafter) for a Faraday rotator should have as low a magnetic saturation as possible. For example, (TbHoBi).sub.3 Fe.sub.5 O.sub.12 is a commonly used BIG film but its magnetic saturation is as large as about 1,000 Oe which is detrimental to miniaturizing of the permanent magnet. For further miniaturization of optical isolators, the magnetic saturation of the BIG should be less than 350 Oe.
One possible way of implementing lower magnetic saturation of a BIG film is to substitute aluminum or gallium for iron. However, substituting aluminum or gallium for iron has adverse effects on the characteristics of optical isolators, such as lower Faraday rotation coefficients, increased temperature dependence of the Faraday rotation coefficients, and increases in magnetic compensation temperature (temperature at which the magnetization becomes zero).
When a BIG film thicker than 500 .mu.m is grown on a non-magnetic garnet substrate by a liquid phase epitaxial method (LPE), the BIG film may be cracked on its periphery or the substrate can break during the growth of crystal. The manufactured BIG film must then be polished into a Faraday rotator. Thus, the BIG film must have an extra thickness of about 30.mu. as a polishing margin.
Therefore, Faraday rotators should have a thickness less than 470 .mu.m and the Faraday rotation coefficient should preferably be larger than 957 degree/cm (=(45/470).times.10,000). Since light having a wavelength of 1.55 .mu.m is used in optical fiber communications, all of the physical properties described in the following description are assumed values at 1.55 .mu.m.
Isolation is an important property that reflects the return loss of an isolator and should preferably be more than 25 dB in the temperature range from -40 to +60.degree. C. The isolation of an optical isolator varies with temperature. This is due to the fact that a Faraday rotation angle has a temperature dependence. In order to achieve an isolation more than 25 dB, a maximum tolerable change in Faraday rotation angle is 3.2.degree.. If a Faraday rotator has an absolute maximum tolerable change of 3.2.degree. in the temperature range from -40 to +60.degree. C., temperature dependence is less than 0.064 degrees/.degree. C. (=3.2/50).
Further, at temperatures below the magnetic compensation temperature of a Faraday rotator, the Faraday rotator does not function as an optical isolator at all since the sign of the Faraday rotation coefficient changes at temperatures below the magnetic compensation temperature. Thus, the magnetic compensation temperature of a Faraday rotator must be below -40.degree. C.
Thus, a low magnetic saturation type Faraday rotator requires a bismuth-substituted rare-earth iron garnet single crystal film having a magnetic saturation of less than 350 Oe, Faraday rotation coefficient larger than 957 degree/cm, temperature dependence of Faraday rotation angle less than 0.064 degree/.degree. C., and magnetic compensation temperature below -40 C. No BIG has been reported which meets all of the aforementioned requirements.